This invention relates to a control system for regulating the wall temperature of a pressure vessel adapted to conduct hot rare gas, particularly helium, from a nuclear reactor.
A pressure vessel of this type which is sometimes referred to in the art as a boiler, receives gases under pressure at a specific and required temperature range. If heat is given up to the gas by indirect heating or by an exothermic chemical reaction taking place in the gas itself, then the temperature of the gas rises. The gas passing through the vessel or boiler is frequently used as a heating medium in which case heat is transferred from the gas to another medium, usually by indriect heat transfer. The heat withdrawn from the gas is used to generate steam or to carry out endothermic chemical reactions. In such instances, the temperature of the gas flowing in the vessel or boiler drops.
Vessles constructed in the manner known in the art to conduct a gas at a high temperature gradient, take the form of a double-jacket vessel. The gas is guided through such a vessel so that the gas, at a relatively cooler temperature, flows through an annular chamber between an outer jacket and an inner jacket which is usually not subjected to pressure. If the low temperature of the gas is above 100.degree. C, it is a conventional practice to apply insulation directly onto the outer skin of the outer pressure jacket.
The temperature of the gas flowing through the inner vessel is frequently so high that the inner vessel is provided with a layer of insulation, such as masonry. The inner vessel may be filled with a catalyst or other ceramic or metallic material so that the total mass thereof provides a considerable heat storage capacity.
If the flow of gas through an externally-insulated pressure vessel is interrupted due to a malfunction, such as a breakdown of a blower or the like, the stored heat by the mass of the inner vessel is transferred or otherwise passed to the surrounding parts. As a result, a temperature equalization occurs. The wall of the outer pressure vessel undergoes a temperature rise beyond the permissible temperature whereby the outer wall of the vessel may tear or fracture. In practice, the risk of vessel failure is frequently encountered due to the expansion of the pressure vessel.
The expansion of hot, particularly combustible gases, is not without risk and requires expensive safety devices. There is also a specific danger if the gas in the pressure vessel is a contaminated rare gas, such as helium, which is heated in a nuclear reactor and remains in communication with the reactor during the malfunction. Expansion of the gas in the atmosphere is impossible because of the risk of contamination and the high cost of the gas. The investment for equipment to rapidly cool a flow of helium from a reactor, purifying the helium and collect it is very high.
In such instances, therefore, it is conventional practice to dispense with external thermal insulation and cool the outer skin of the pressure vessel. To this end, the wall of the pressure vessel is provided with, for example, cooling ducts or cooling coils which are welded onto the wall for the passage of cooling water or a cooling gas. The loss of energy during normal operation of the vessel due to the flow of heat in the cooling medium is considerable but is acceptable for safety reasons.